The present invention relates to interconnection structures for joining an electronic array module to a printed circuit board or card, and more particularly to a structure for forming solder interconnection joints that exhibit reduced stress resulting from thermal mismatch of the module and the printed circuit board. The present invention is especially suitable in providing micro ball grid arrays (xcexcBGA). The present invention is also concerned with the method for fabricating such interconnection structures.
Micro ball grid array (xcexcBGA) refers to a surface mounting packaging technique whereby the solder ball array pattern is typically less than about 0.75 mm pitch. This is contrasted to conventional ball grid array technology whereby the pitch is at least about 1.0 mm. Pitch refers to the distance measured from the center of an adjoining solder ball to the center of the next adjacent solder ball.
In second level packaging of an electronic array module or chip carrier to a printed circuit board or card, the solder interconnection is rigid and cannot flex under thermal mismatch conditions. As ball to ball spacing shrinks to 0.75 mm pitch and less, the diameter of the ball must be reduced to achieve such spacing. This results in a lower stand off between the module and printed circuit board or card, resulting in less resilience to stress during expected thermal cycles due to the thermal mismatch between the module and the printed circuit board. The reliability of such a second level package interconnect is therefore jeopardized due to the loss of resilience to stress. Accordingly, xcexcBGA has seen limited industrial applications because of reliability concerns. Reliability can be addressed by employing epoxy compositions to adhere the xcexcBGA package to integrated circuit boards or cards in order to minimize the effects of thermal mismatch between the device and the circuit board or card. However, this is not especially satisfactory since the epoxy compositions cannot be reworked in that they are thermosetting polymer materials.
An object of the present invention is to minimize the problem from thermal mismatch between electronic array modules surface mounted on a printed circuit board or printed circuit card. According to the present invention, the reliability is significantly increased. More particularly, the present invention is concerned with a solder interconnection for forming connection between a module or chip carrier and printed circuit board or printed circuit card that comprises a plurality of solder connections arranged in a micro area grid array that joins solder wettable pads on a major surface of the module to a corresponding set of solder wettable pads of the printed circuit board or card. According to the present invention, the solder connections are column shaped with the height of each connection being at least about 1.4 times its diameter. By employing the required column shaped interconnections specified by the present invention, the stand off distance between the module and printed circuit board or card is significantly increased. This in turn increases the resilience of the connection during stress experienced during thermal cycles due to the thermal mismatch between the module and printed circuit board or card.
The present invention is also concerned with the method for interconnecting a module to a printed circuit board or card which comprises attaching the circuit module to the printed circuit board or card by a plurality of solder interconnections that extend from solder wettable pads on a major surface of the module to a corresponding set of solder wettable pads of the printed circuit board or card. The solder connections are column shaped with the height of each of the connections being at least about 1.4 times its diameter and the solder connections are arranged in a micro area grid array.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.